From 54ccc8b588e175397eb1f23416100b40fb1196b0 Mon Sep 17 00:00:00 2001 From: Jilles Tjoelker Date: Sun, 24 Mar 2013 22:48:45 +0000 Subject: [PATCH] sh(1): Mention possible ambiguities with $(( and ((. In some other shells, things like $((a);(b)) are command substitutions. Also, there are shells that have an extension ((ARITH)) that evaluates an arithmetic expression and returns status 1 if the result is zero, 0 otherwise. This extension may lead to ambiguity with two subshells starting in sequence. --- bin/sh/sh.1 | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/bin/sh/sh.1 b/bin/sh/sh.1 index 09d94f3a78bb..84dcdde27808 100644 --- a/bin/sh/sh.1 +++ b/bin/sh/sh.1 @@ -32,7 +32,7 @@ .\" from: @(#)sh.1 8.6 (Berkeley) 5/4/95 .\" $FreeBSD$ .\" -.Dd January 22, 2013 +.Dd March 24, 2013 .Dt SH 1 .Os .Sh NAME @@ -1061,6 +1061,9 @@ A subshell environment may be implemented as a child process or differently. If job control is enabled in an interactive shell, commands grouped in parentheses can be suspended and continued as a unit. .Pp +For compatibility with other shells, +two open parentheses in sequence should be separated by whitespace. +.Pp The second form never forks another shell, so it is slightly more efficient. Grouping commands together this way allows the user to @@ -1618,6 +1621,16 @@ and .Ic times returns information about the same process if they are the only command in a command substitution. +.Pp +If a command substitution of the +.Li $( +form begins with a subshell, +the +.Li $( +and +.Li ( +must be separated by whitespace +to avoid ambiguity with arithmetic expansion. .Ss Arithmetic Expansion Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value.